1. Field of the Invention
The present invention relates to an optically controlled optical-path-switching-type data distribution apparatus and distribution method. More specifically, the present invention relates to an optically controlled optical path switching type data distribution apparatus and distribution method using an optical packet communication system to distribute a large volume of digital data such as high definition image data, high definition moving image data, etc. from a server to a specific client among a plurality of clients in an office of a company, a plant, a hospital, an ordinary home, etc.
2. Description of the Invention
Due to the dramatic progress of personal computers and their peripheral equipment, an almost inconceivably large volume of digital data including digitized and recorded high-definition static and moving images is transmitted everyday. For example, when a conventional color television program is digitized and recorded on a hard disk type storage unit in a personal computer or on an optical recording medium such as a DVD at home, a large volume of digital data with several gigabytes of data per one airtime hour is handled. Furthermore, with the advancement and improvement of diagonal medical instruments, super high-definition static images and digital high-vision images represented by 1600×1200 pixels or more are used, for example, to detect a cancer in early stages. An example is that a cancerous growth as small as 1 mm, or even smaller, in an early stage can be detected by rendering a three dimensional image by combining 1000 sliced images of a human body, each image having 1600×1200 pixels, on a display.
Recently, because the capacity of typical hard disk type storage units has increased, the recording of such high definition static images and moving images is not highly problematic. However, quick transfer or transmission of an image data to a remote site (another room within a hospital, a classroom in a university hospital and another site for remote treatment) still requires several minutes to several tens of minutes, even when using a high speed LAN capable of communication at 1 gigabit per second. Therefore, transfer of such a large volume of digital data to an optical recording medium such as DVD first has been practiced, however, a significant time is required to record the data and to physically transport the recording medium. However, as medical use requires electromagnetic compatibility, it is desirable that a large volume of digital data can be transmitted in the form of optical signals from a transmitting device provided with an electromagnetic shield to a receiving device also provided with an electromagnetic, without employing any electric signals during the transmission path. Such a configuration would be preferable in order to minimize generation of electromagnetic waves associated with transmission and reception of the high speed digital data, as well as switching of transmission paths, and further to eliminate interference from electromagnetic waves originating from external sources.
Currently, mass-produced optical transceivers having a data communication speed on the order of 10 to 40 Gbps per second are available and are successfully operated in data distribution devices in which their data transmitting and receiving sides are connected via optical fiber. In the field of data delivery using optical signals, the most strongly desired technological development is an optical path switching unit (optical switch), or light-to-light direct switching unit that does not employ an electric signal for use in high speed distribution of data from a data distribution device (server) to specific client devices.
Known apparatuses and methods for switching the path of light traveling through space (optical paths) include, for example, a space division type that switches optical paths in an optical waveguide or between optical waveguides, a wavelength division multiplexing type that switches a multiplexed light beam having a plurality of wavelengths by dividing the light beam for optical paths according to the wavelength, a time division multiplexing type that switches optical paths of light beams that is time-division-multiplexed at an constant time interval and a free space type that divides and couples spatially optical paths of light beams propagating through space using a mirror or a shutter. Each of these schemes can be multiplexed or a plurality of different schemes can be used in combination.
Proposed space-division-type optical switches include those that utilize a directional coupler, those that create a copy of an optical signal using an optical dropper and switch a light beam between ON and OFF using a gate device, those that transmit or reflect a light beam propagating a waveguide by varying the refractive index of the waveguide at a crossing portion of an intersection or a Y-shaped branching point, and others. However, all of these remain in the stage of research and development. Apparatuses employing a thermo-optical effect created by using an electric heater to vary the refractive index of a waveguide of a Mach-Zehnder-interferometer-type optical waveguide switch are approaching practical application, but such apparatuses are disadvantageous in that this type of apparatus has a low response speed, of approximately 1 millisecond, and also requires an electric signal to operate the optical switch.
Meanwhile, available free-space-type optical switches include a micro-electro mechanical system (abbreviated to MEMS), an exciton absorption reflection switch (abbreviated to EARS), a multi-stage-beam-deflector-type optical switch, a hologram-type switch, a liquid crystal switch, and others. However, these switches cannot be said to be sufficiently developed for practical use because they have assignments such as that they have mechanically movable portions; they are dependent on polarized electromagnetic radiation, and other factors.
On the other hand, there is active study of total-light-type optical devices or optical control methods that modulate the intensity or the frequency of a light beam directly by utilizing variation of the transmittance or the refractive index caused when an optical device is irradiated with light. The inventors of the invention described in the present application are continuing an ongoing study of an optical control method aimed at development of a new information processing technique with a total-light-type optical device, etc. using an organic nanoparticle thermo-optical lens forming device formed by dispersing organic pigment aggregate in a polymer matrix (see Takashi Hiraga, Norio Tanaka, Kikuko Hayamizu and Tetsuo Moriya, “Formation, Structure Evaluation and Photo-Material Property of Associated/Aggregated Pigment”, Journal of Electronic Technology General Institute, Electronic Technology General Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Vol. 59, No. 2, pp. 29-49 (1994)). Currently, a device employing a scheme that modulates a signal light beam (780 nm) by a control light beam (633 nm), having a characteristic that the control light beam and the signal light beam are coaxial and have incidence of the same focal point, and based on an operational principle that the signal light beam is refracted by a thermal lens formed transiently by absorption of the control light beam, is being developed and a high-speed response of approximately 20 nanoseconds has been achieved. Japanese Patent Application Laid-Open Publications Nos. 1996-286220, 1996-320535, 1996-320536, 1997-329816, 1998-90733, 1998-90734 and 1998-148852 disclose an optical control method of carrying out intensity modulation and/or light flux density modulation of a signal light beam transmitted through an optical device by reversibly varying the transmittance and/or the refractive index of the signal light beam in a different wavelength band from that of the control light beam by irradiating the optical device comprising optically responsive composition, with the control light beam, wherein the control light beam and the signal light beam are converged and irradiated on the optical device, and the optical paths of the control light beam and the signal light beam are adjusted such that an area having the highest photon density in the vicinity of a focus (beam waist) of each of the control light beam and the signal light beam are overlapped on each other in the optical device. Furthermore, Japanese Patent Application Laid-Open Publication No. 1998-148853 discloses an optical control method of carrying out intensity modulation and/or light flux density modulation of a signal light beam transmitting a thermal lens by reversibly forming the thermal lens based on the distribution of density variation caused by a temperature increase generated in an area of the photo-responsive composition, that has absorbed the control light beam and the surrounding area thereof, wherein a control light beam and the signal light beam having a wavelength different from each other are irradiated on an optical device comprising photo-responsive composition, the wavelength of the control light beam is selected from a wavelength band that the photo-responsive composition absorbs. Yet further, in Japanese Patent Application Laid-Open Publication No. 1998-148853, it is described that a pigment/resin film or a pigment solution film is, for example, used as the optical device and a response time of the signal light beam against the irradiated control light beam for the case where the control light beam has a power of 2 to 25 mW is shorter than 2 μsec.
Here, the thermal lens effect is a refractive effect in which molecules, etc. that have absorbed light in the central area of light absorption convert the light into heat, a temperature distribution is created by propagation of this heat to the surrounding area, and, as a result, the refractive index of an optical transmitting matter is varied spherically from the center of the light absorption to the outer region to create a distribution for the refractive index which is lower at the center of the light absorption and higher continuing outward, with functions similar to those of a convex lens. The thermal lens effect has long been utilized in the field of spectral analysis, and an ultra high sensitivity spectral analysis can be carried out that can detect the light absorption of even a single molecule (see Kitao Fujiwara, Keiichiro Fuwa and Takayosi Kobayasi, “A Laser-Induced Thermal Lens Effect and Its Application to Calorimetry”, Chemistry, Kagaku-Dojin, Vol. 36, No. 6, pp. 432-438 (1981); Takehiko Kitamori and Tsuguro Sawada, “Photo-Thermo Conversion Spectral Analysis Method”, Bunseki, Japanese Society of Analytical Chemistry, March, 1994, pp. 178-187).
Moreover, Japanese Patent Application Laid-Open Publication No. 1985-14221 discloses, as a method of deflecting an optical path using variation of refractive index caused by the thermal lens effect or heat, a method of deflecting a light beam by creating a distribution of refractive index in a medium by providing heat using a heating resistor.
However, because, in all of the above methods, heat is produced using a heating resistor and a medium is heated using conduction, these methods have an intrinsic problem of diffusion of heat. That is, because of the diffusion of heat, a fine thermal gradient cannot be provided over a large area and a desired distribution of the refractive index cannot not be easily or reliably obtained. Furthermore, in actual practice, the fine processing of a heating resistor is limited, even when a photolithography technique used for semiconductor integrated circuits is employed, such that it is not possible to prevent the size of the device from increasing. When the size of the device increases, the optical system becomes larger and more complicated. Furthermore, because heat is produced using a heating resistor and the medium is heated by conduction of the heat, this invention has intrinsic disadvantages such as that the response is slow and the frequency for varying the refractive index cannot be increased.
Moreover, Japanese Patent Application Laid-Open Publication No. 1999-194373 discloses a deflecting device using an optical device, comprising at least the optical device comprising an photo-sensitive composition and intensity distribution adjusting means for irradiating the optical device with light in a wedge-shaped optical intensity distribution, wherein a distribution of refractive index is formed in the optical device by a control light beam and deflection of a signal light beam having a wavelength different from that of the control light beam is carried out by the distribution of the refractive index. Although this scheme is excellent in terms of controlling light using light, this scheme is constrained in that the angle of deflection must be within 30 degrees and, therefore, is problematic in that directions for switching optical paths cannot be freely set.
The present inventors disclosed in an earlier patent application an optical path switching apparatus and optical path switching method with no polarized-electromagnetic-wave dependence, for which angles and directions for switching optical paths can be set freely, with which optical intensity attenuation of a signal light beam is small, and which can be used in a plurality of connection; a control light beam having a wavelength selected from a wavelength band that a light absorbing layer film absorbs and a signal light beam having a wavelength selected from a wavelength band that the light absorbing layer film does not absorb are respectively converged and irradiated on the light absorbing layer film in a thermal lens forming device containing at least the light absorbing layer film; arrangement is adjusted such that at least the control light beam is focused within the light absorbing layer film; and a thermal lens based on a distribution of the refractive index created reversibly caused by a temperature increase produced in an area of the light absorbing layer film that has absorbed the control light beam and the area surrounding the area is used. Thereby, a state wherein the converged signal light beam exits from the thermal lens forming device at an ordinary divergence angle when the control light beam is not irradiated and no thermal lens is formed, and another state wherein the converged signal light beam exits from the thermal lens forming device at a divergence angle larger than the ordinary divergence angle when the control light beam is irradiated and a thermal lens is formed are realized in response to the presence or absence of the irradiation of the control light beam; when the control light beam is not irradiated and no thermal lens is formed, the signal light beam exiting from the thermal lens forming device at the ordinary divergence angle is caused to travel, either unchanged or with its ordinary divergence angle modified using a light-receiving lens, straight through a hole in a mirror. On the other hand, when the control light beam is irradiated and a thermal lens is formed, the signal light beam exiting while diverging from the thermal lens forming device at a divergence angle larger than the ordinary divergence angle is reflected, unchanged or after the divergence angle of the divergence is changed using a light-receiving lens, using the hole-provided mirror. (See Japanese Patent Applications No. 2002-275713 and No. 2004-44991.)